You can anchor other assets as well of course but digital currencies will allow you to be accessible worldwide and potentially not have to bother with customer accounts.
You could easily anchor bitcoin, ethereum, zcash etc. Obviously these digital currencies are already easy to send to anyone in the world so there is less of a reason than to anchor things like USD or shares of some company but there are still advantages to anchoring digital currencies. In some cases they will move much faster on Stellar. But most importantly it allows you to trade the digital currencies with each other and with whatever other assets are on Stellar.
There are many different ways to provide this service. Here are a couple ideas. I'll use bitcoin when discussing this but it can work basically the same for most other digital currencies.
1) people depositing coins with you
Option1 You provide a page where they can enter in their Stellar account. You check to make sure this stellar account trusts your anchor. If everything is ok then you provide them a bitcoin address. You monitor this bitcoin address. Any BTC sent to this address triggers you to send BTC in Stellar to the stellar account they provided you.
Option2 You publish a particular bitcoin address that you control and then any bitcoin payment to that address will cause you to send BTC in Stellar to the Stellar account that was put in the op_return.
2) people withdrawing coins from you
Ideally it would be very simple with everything embedded in the Stellar transaction so there doesn't have to be a UI. Also it would be nice for privacy if the sending Stellar account wasn't tied publicly to the receiving bitcoin account.
Option1 You create a Stellar account that you monitor. Any Stellar BTC that is sent to this account causes you to send actual BTC to the address that is in the memo. Downside here is that the sender would have to use memo_hash in the Stellar transaction to store the bitcoin address and Stellar clients don't really support this. But if you are also making the Stellar client that they are using this might be a good option. A better twist on this method is Option4 below
Option2 You provide some page that allows them to link a given Stellar account with a payout bitcoin address. With this method you will have to keep this association in your DB. You will also need to make sure that the Stellar account actually authorized the payout bitcoin address. You don't want people hijacking other people's payout addresses. A couple ways to do that:
2a) Have them sign the request with their Stellar public key that is sent to your server that associates the bitcoin address with the Stellar address.
2b) The page allows them to enter in a bitcoin address and it gives them a Stellar account and a random small amount of lumens to send to this address. If they send the right amount of lumens then the bitcoin address will be associated with the sending Stellar account.
Option3 You have a Stellar account that you monitor. When Stellar BTC is sent to this account you look up the value of "btc_address" in the account data fields of the sending account. Then you send actual BTC to the address that is associated with the sending Stellar account. Downside again is not good client support yet for setting account data.
Option4 Federation payout. Allow them to send payments to any bitcoin address just through federation. For example like: 1FoXbDsvXgwVoFwkszaZjkuRRDADoWTygX*anchor.com
Your federation server would make an ID for each bitcoin address that was requested this way and return that ID in the federation response. Saving the mapping between that ID and the bitcoin address. Later when a payment comes in with that ID in the memo you know to send to that bitcoin address that was saved in the DB.
This is the simplest method from a user experience point of view. You can even imagine clients allowing you to specify your "Bitcoin Anchor" somewhere and then if you enter a bitcoin address in the send dialog it knows you are sending bitcoin and knows to do federation on the address with the given anchor as the domain.
An example of a site anchoring BTC: naobtc.com
